1. Field of the Invention
The invention relates to a photoelectric element in a solid-state image pickup device in which a large number of photoelectric elements and scanning circuits for taking out optical information of the photoelectric elements are integrated on a semiconductor substrate.
2. Description of the Prior Art
The principle of such a solid-state image pickup device is described in detail in a paper by Savvas G. Chamberlain, entitled "Photosensitivity and Scanning of Silicon Image Detector Arrays", reported in IEEE Journal of Solid-State Circuits. Vol. SC-4, No. 6, December 1969, pp. 333-342. The device described therein is made up of metal-oxide insulated-gate semiconductor transistors (hereinbelow, abbreviated MOSTs) which are arranged in the form of a matrix on a silicon substrate. The source of the MOST and the substrate constitute a photodiode. Light incident on the photodiode creates charges, and the charges are stored in the capacitance of the photodiode. The charges thus stored are read out to a signal output line connecting the drains of the MOSTs in common, by driving the MOSTs by means of scanning circuits for X- and Y-directions.
A solid-state image pickup device should have a resolution power common for image pickup tubes which are used in current television broadcast circuits, and it therefore requires 500 .times. 500 photoelectric elements as well as corresponding switches for x-y addressing and scanning circuits each having 500 stages for turning the switches "on" and "off". Accordingly, the device is ordinarily fabricated with MOS. LSI technology with which a high density of integration is comparatively easily achieved.
Since the solid-state image pickup device constructed as described above can have the photoelectric elements and the switches fabricated in a monolithic structure, it is suited for integration. Since it uses the MOSTs, it has the advantage of low power dissipation. The degree to which the photodiodes can be integrated is accordingly comparatively large; yet, it is to the extent of an array of 100 .times. 100. The solid-state image pickup device of this type involves serious problems as mentioned below.
1. Charges created by the incident light are stored in the capacitance of the diode. In order to attain the required signal-to-noise ratio, a predetermined diode capacitance is necessary. The capacitance of the junction diode is generally 1 pF for each 100 .mu.m square and a value of 50 .mu.m square is needed in order to obtain the required signal-to-noise ratio. In the case of integrating an array of 500 .times. 500 diodes, the IC chip size of the image pickup devices reaches 25mm square. Considering that the present-day size is 2-3mm square in an ordinary chip and 7mm square even in the greatest chip, fabrication of the IC of 25mm square is extremely difficult.
2. As is well known, an IC photo-mask is required in order to fabricate an image pickup IC. The size of a chip which can be formed by one-shot photography at present is 10mm square. A chip of 25mm square must accordingly be fabricated by a method wherein the chip size is divided in two in each of x- and y-directions, i.e. on the whole into four blocks, which are photographed separately and then joined to one another. This complicates the manufacture of the masks. Moreover, an image obtained by the image pickup device produced by using the divided masks includes white (or black) lines corresponding to the joints which run longitudinally and laterally, and the picture quality is degraded.